Production of non-oriented ferrous magnetic materials



Dec. 31, 1963 v. w. CARPENTER ETAL 3,116,179

PRODUCTION OF' NON-ORIENTED FERROUS MAGNETIC MATERIALS Filed Sept. 27, 1960 .N .OWN Qu\wmu Q m, N N

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United States Patent O PRDUCTIN @F NGN-(PRIENTED FERRUUS MAGNETHC MATERIALS Victor W. Carpenter and .lohn M. Jackson, Middletown,

Chio, assignors to Armco Steel Corporation, Middletown, Ghia, a corporation of Ghia Filed Sept. 27, 1960, Ser. No. 58,715 5 Claims. (Ci. 14S-12.1)

The invention relates to the production of ferrous strip or sheet materials by a process which includes the basic steps of hot rolling the material to an intermediate gauge, decarburizing the material by annealing it with the hot mill scale still on its surfaces, cleaning the surfaces of the material, and then cold rolling it to gauge. While not necessarily limited to the production of electrical steels, processes including these basic steps are particularly useful in the production of low carbon steels and silicon-iron sheets or strips for magnetic uses. Since the materials are usually non-oriented, they have numerous electrical uses, as for example in rotating electrical machinery and in certain types of transformers, in which uses core laminations are formed by stamping or diecutting. Consequently the die life of the material is of importance.

The chemistry of the material is not a limitation on the invention, excepting as to the carbon content; and the material may range from low carbon steels to silicon-iron alloys containing from about 0.5% to about 3.5% silicon. lt has long been known that annealing such materials in an intermediate gauge hot rolled condition with the hot mill scale or oxide still on their surfaces will result in an effective decarburization. The decarburization occurs through the interaction of the surface oxide with the carbon in the steel. When the intermediate hot rolled gauge is about 0.06 to .10 in. in thickness, and the initial carbon `content about 0.05% to 0.10%, a box anneal in the presence of the mill scale can be depended upon to reduce the carbon content to a value less than about .01% yas taught in U.S. Patent No. 2,236,519.

During the decarburization, the mill scale is reduced or partially reduced, leaving on the surfaces of the stock a layer of substance which is exceedingly' difcult to remove either by abrasion or by pickling or both. But it is not practicable to leave this layer in place during the subsequent cold rolling not only because improper surface characteristics may be attained but also because the material will have a poor die life and will be unsuitable for punching.

lt is a fundamental object of the invention to provide a process including the basic steps set forth above, which will be devoid of the above defects.

In prior art practices, coils of hot rolled ferrous material bearing the hot mill scale have been given box anneals, the air being generally liushed from the box, and the atmosphere remaining being that which is generated by the action of the carbon from the steel with the scale on the strip. As indicated, the decarburization is accompanied by a reduction of some or all of the iron oxide in the scale, leaving generally a dispersion of oxide in a matrix of iron, and in the case of siliconiron materials, a dispersion of silica or silicates along with iron oxide in a matrix of iron. It has been known that if the decarburized material is then reoxidrized, the surface Ilayer is converted into a form in which it may more easily be abraded, blasted, or pickled from the strip. However, this involves decoiling the strip, and giving it a continuons or open anneal in an oxidizing atmosphere. In other words, a second anneal is involved, which requires additional equipment and additional expense.

It is an object of the invention to provide a method in which a single box annealing step can be relied upon not only to decarburize the stock, but also to leave upon the surfaces of the stock a layer of material which can readily be removed by the known methods of preparing undecarburized hot rolled strip for cold rolling.

These and other objects of the invention which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, are accomplished by' that procedure of which certain exemplary embodiments will now be described. Reference is made to the accompanying drawings wherein:

FIG. 1 is a diagram in which the ratio of hydrogen to water vapor in the annealing latmosphere is plotted against temperature.

FIG. 2 is a diagram in which the ratio 0f carbon dioxide to carbon monoxide is plotted against temperature in the `annealing operation.

As has been indicated, one of the advantages of this invention is the provision of a box anneal which will be effective both for decarburization and for oxidation of the materials remaining upon the surfaces of the stock, so that this layer may be readily removed. One of the objects of the invention is to accomplish both operations at a minimum of cost. One way of accomplishing the objects of the invention is to control the atmosphere during the box anneal in such a way that it is oxidizing to carbon but reducing to iron. While the decarburization will 4take place whether the atmosphere be oxidizing, reducing, or neutral, the decarburization generally occurs in a lesser time if the atmosphere is initially controlled as indicated. When the decarburization is complete, the atmosphere may then be changed so as to oxidize the reduced scale layer on the strip.

Another way in which the objects of the invention can be accomplished is to control the atmosphere so that, while the steel will be decarburized, the surface layer of materials will remain in the oxidized form during the entire cycle. This also has been found to give a surface layer which is readily removed.

The practice of the process in either of these ways, however, requires that the surfaces of the steel be open to the atmosphere of the furnace throughout the entire annealing operation, and that there be movement of the annealing atmosphere over all parts of the surfaces of the steel. An open or continuous anneal is not suitable for the practice of this invention, for the reason that it does not permit a suilicient soaking at temperature. The steel should be box annealed in coils. Tightly wound coils do not permit the practice of this invention because of the limited access of the atmosphere to the interior convolutions. However it is possible to prepare coils in such a way as to permit all portion of the surfaces of the convolutions to be bathed in the annealing atmosphere. Coils may be wound with narrow corrugated metal strips located at their edges so as to separate the convolutions. Also coils may be wound with wire or nylon strands or the like between the convolutions, these intermediate members being removed before or after the coils have been annealed. The convolutions will then stand apart from each other so as to permit the annealing atmosphere to pass between them. Furthermore, it is possible so to design a box or furnace that the annealing atmosphere will be blown toward one end of each coil so as to pass continuously between the convolutions and keep all parts of the coil surfaces bathed in a substantially constant composition of atmosphere.

Various gases may be used as the annealing atmosphere, including air where constant oxidizing conditions are to be maintained. Where it is desired to control the atmosphere in the first part of the annealing cycle in such a way that it is oxidizing to carbon but reducing to iron,

gases having a reducing potential will be used. There are many such gases including partially coinbusted hydrocarbon gases, cracked ammonia and various mixtures of hydrogen, nitrogen and water vapor. Irrespective of the specic gas or gaseous mixture which is employed, the reducing potential will normally he derived either from a hydrogentwater vapor ratio, or from a carbon dioxide: carbon monoxide ratio, or both. In order to guide the worker in this art, varying hydrogen: water vapor ratios have been plotted in FIG. 1 against annealing temperatures. The curves demark areas which are respectively designated as Fe, FeO and Fe304. In the area marked Fe the conditions shown on the chart are reducing toward, iron, while the other areas indicate conditions which are oxidizing toward iron in a greater or less degree. Similarly in FIG. 2 where the carbon dioxidezcarbon monoxide ratios have been plotted against temperature, like areas indicate conditions which are reducing or oxidizing toward iron.

The actual conditions of operation can be determined in accordance with the economics of the process. Ternperatures and atmospheres can be chosen such that decarburization is most rapid while scale formation is at a minimum. Preferred temperatures of operation lie between about l300 F. and 1600 F.

Example I Silicon-iron containing about 2.19% silicon and 0.029% carbon was decarburized for 2 hours at 1500 F. in a mixture of hydrogen, nitrogen and water vapor. The HQI/H2O ratio was 0.2. The nal carbon Content Was 0.0054% and the surface was fully oxidized and readily blasted.

Example Il The same starting material was decarburized to 0.0069% carbon under the same conditions except that the liz/H2O ratio was 4.0. A layer of reduced iron remained on the surface of the material. Reheating for 2 hours at 1300 F. in an atmosphere having a Hz/HZO ratio of 0.2 caused the surface to be almost completely reoxidized. A similar treatment at l500 F. resulted in complete reoxidization. In commercial practice it is preferred to avoid a change in the fig/H2O or CO2/CO ratios during the decarburizing step by using a low Iig/H2O ratio and a high CO2/CO ratio throughout the anneal.

Under these circumstances it will be advantageous to adjust the atmospheres from neutral or reducing to oxidizing during the anneal, or to maintain the same atmosphere but change the temperature to produce an oxidizing atmosphere during the latter part of the anneal.

In either event, at the conclusion of the process the steel will be found to have been decarburized and its surfaces will be covered with an oxidized layer which can readily be removed by pickling, abrasion, blasting or any combination of these procedures.

If decarburization under oxidizing conditions is practiced, the anneal is carried on in those areas of FlGS. l and 2 denoted by the designations Fe() or Fe304. In these areas, the greater the oxidizing potential of the gas the lower the carbon will be in the finished product.

The invention has been disclosed in connection with a process involving the basic steps of hot rolling, decarburizing, and cold rolling, with an intermediate cleaning or pickling of the material between the decarburization step and the cold rolling step. It will be understood that the principles of the invention 4may be carried out in more elaborate processes, and that the spirit of the invention is not violated by the addition of other steps, either during the hot rolling, or more particularly following the cold rolling to a gauge which is either the final gauge or close to it. For example, the addition of final heat treatments to improve the magnetic qualities of the material may be practiced without departing from the invention. Also it is Within the purview of the invention to subject the decarburized and oxidized material to further hot rolling with or without subsequent cold rolling.

While the term box annealing has been employed herein, it is to be understood that it is not confined to the use of a metal annealing box covering the coils of iaterial to be treated, the whole being enclosed in a furnace. It is intended to be construed broadly enough to include the use of a muffle furnace which serves in lieu of both the box and the exterior furnace elements in which the box is used. Such muille furnaces may be electrically heated, or preferably heated by radiant tubes containing the products of combustion from burners. In some instances the products of burner combustion may be introduced directly into the muflle and may be so controlled as to give the desired effects upon the material being treated.

The invention having been described in certain exemplary embodiments, what is claimed as new and desired to be secured by Letters Patent is:

l. A process of producing low carbon ferrous strip 0r sheet materials consisting essentially of the steps of hot rolling ferrous material to an intermediate gauge, box annealing the material with the hot mill scale still on its surfaces whereby to effect solid state decarburization of the strip through the reaction of carbon therein with the iron oxide of the said hot rnill scale on the surfaces of the strip, cleaning the surfaces of the material and then cold rolling the material to gauge, wherein the hot rolled material is annealed in a single Inutile and in open coils all surfaces of which are bathed by an annealing atmosphere, and in which the annealing atmosphere is controlled so as to be oxidizing to iron at least at the end of the annealing treatment, whereby the material is decarburized and the product is left with a surface coating consisting substantially of oxides of iron which is easily removed by cleaning operations.

2. The process claimed in claim 1 wherein the ferrous material has an initial carbon content not above about 0.10% and a final carbon content not in excess of about 0.01%.

3. The process claimed in claim 1 wherein the ferrous material is a silicon-iron alloy containing from about .5% to about 3.5% silicon.

4. The process claimed in claim l in which the entire annealing treatment in the box is carried on in an atmosphere which is oxidizing to iron, whereby the surfaces of the material continue to be covered by an oxide layer.

5. The process claimed in claim 2 wherein at the iirst part of the box annealing treatment the annealing atmosphere is so controlled that it is oxidizing to carbon but reducing to iron, and in which at the end of the annealing treatment the conditions of temperature and atmosphere composition are so controlled as to re-oxidize iron previously reduced from the hot mill scale on the surfaces of the material.

References Cited in the iile of this patent UNITED STATES PATENTS 2,236,519 Carpenter Apr. l, 1941 2,287,467 Carpenter et al June 23, 1942 2,455,632 Williams Dec. 7, 1948 2,533,394 Oppenheimer Dec. 12, 1950 OTI-IE c REFERENCES Physical Chemistry of Metals, pp. 217-222, Darten and Gurry, McGraw-Hill Book Co., Inc., 1953. 

1. A PROCESS OF PRODUCING LOW CARBON FEROUS STIP OR SHEET MATERIALS CONSISTING ESSENTIALLY OF STEPS OF HOT ROLLING FERROUS MATERIAL TO AN INTERMEDIATE GAUGE, BOX ANNEALING THE MATERIAL WITH THE HOT MILL SCALE STILL ON ITS SURFACES WHEREBY TO EFFECT SOLID STATE DECARBURIZATION OF THE STRIP THROUGH THE REACTION OF CARBON THEREIN WITH THE IRON OXIDE OF THE SAID HOT MILL SCALE ON THE SURFACES OF THE STRIP, CLEANING THE SURFACES OF THE MATERIAL AND THEN COLD ROLLING THE MATERIAL TO GAUGE, WHEREIN THE HOT ROLLED MATERIAL IS ANNEALED IN A SINGLE MUFFLE AND IN OPEN COILS ALL SURFACES OF WHICH ARE BATHED BY ANANNEALING ATMOSPHERE, AND IN WHICH THE ANNEALING ATMOSPHERE IS CONTROLLED SO AS TO BE OXIDIZING TO IRON AT LEAST AT THE END OF THE ANNEALING TREATMENT, WHEREBY THE MATERIAL IS DECARBURIZED AND THE PRODUCT IS LEFT WITH A SURFACE COATING CONSISTING SUBSTANTIALLY OF OXIDES OF IRON WHICH IS EASILY REMOVED BY CLEANING OPERATIONS. 